This invention is directed at control of the temperature of a refrigeration system where the refrigerant mixture includes components whose normal boiling points vary by at least 50 C from the warmest boiling component to the coldest boiling component. More particularly this invention is intended for use in refrigeration systems where the refrigeration effect takes place at temperatures below xe2x88x9270 C.
Refrigeration systems have been in existence since the early 1900s, when reliable sealed refrigeration systems were developed. Since that time, improvements in refrigeration technology have proven their utility in both residential and industrial settings. In particular, very low temperature refrigeration systems, colder than xe2x88x9240 C, currently provide essential industrial functions in biomedical applications, cryoelectronics, coating operations conducted in a vacuum (i.e. physical vapor deposition), and semiconductor manufacturing applications, control of chemical reactions and pharmaceutical manufacturing processes. Another application involves thermal radiation shielding. In this application large panels are cooled to very low temperatures. These cooled panels intercept radiant heat from vacuum chamber surfaces and heaters. This can reduce the heat load on surfaces being cooled to lower temperatures than the panels. Yet another application is the removal of heat from objects being manufactured. In some cases the object is an aluminum disc for a computer hard drive, a silicon wafer for an integrated circuit, or the material for a flat panel display. In these cases the very low temperature provides a means for removing heat from these objects more rapidly than other means, even though the object""s final temperature at the end of the process step may be higher than room temperature. Further, some applications involving, hard disc drive media, silicon wafers, or flat panel display material, involve the deposition of material onto these objects. In such cases heat is released from the object as a result of the deposition and this heat must be removed while maintaining the object within prescribed temperatures. Cooling a surface like a platen is the typical means of removing heat from such objects.
This invention relates to refrigeration systems which provide refrigeration at temperatures between xe2x88x9240 C and xe2x88x92220 C by use of a secondary cooling fluid or coolant. The temperatures encompassed in this range are variously referred to as low, ultra low and cryogenic. For purposes of this application the term xe2x80x9cvery lowxe2x80x9d or xe2x80x9cvery low temperaturexe2x80x9d is used to mean the temperature range of xe2x88x9240 C to xe2x88x92220 C.
A basic requirement of very low temperature refrigeration is that there is no freezout of the individual refrigerant components in the mixture. Refrigerant freezeout will cause blockage of essential refrigerant flow paths in throttle devices and heat exchangers. This leads to a low pressure at the suction side of the compressor and causes the refrigeration process to shut down by virtue of its control circuits. A method of limiting the coldest temperature of a refrigeration process is needed to eliminate refrigerant freezeout.
In many applications, such as the semiconductor device manufacturing industry, it is necessary that refrigeration systems provide very low temperature refrigeration to highly viscous industrial coolants. Highly viscous coolants become very difficult to pump if their temperature becomes too low. Therefore limiting the coldest refrigerant temperature to which the coolant exchanges heat can prevent the coolant from becoming excessively viscous.
Such very low temperatures are needed for a variety of industrial applications. In the semiconductor industry such very low temperatures are important for processing semiconductor wafers. In one such example, the deposition of material on a wafer causes heat to be rejected to the wafer, which heat must be removed. Further, such processes must take place within a specified temperature range. Frequently, the process design requires cooling temperatures of xe2x88x9220 C or colder to achieve desired process conditions. Additionally, very low temperature cooling is needed when the completed wafers are tested.
Traditional refrigeration systems use a device known as a pressure regulating valve that controls the pressure exiting an evaporator. When applied at the outlet of an evaporator it is referred to as a evaporator pressure regulating valve. Such a valve is located downstream of the evaporator. Because conventional refrigeration systems use single component refrigerants, or mixtures of refrigerants consisting of components with normal boiling points within a 30 C span that have been selected to emulate single refrigerants, there is a very close relationship between the evaporating refrigerant pressure and the evaporating refrigerant temperature. Thus, controlling the pressure directly controls the evaporating temperature. On such conventional systems the pressure regulating valve is located at the outlet of the evaporator and is exposed to the evaporator temperature. Since these temperatures are typically xe2x88x9240 C or warmer, there are no significant restrictions in material selection, and elastomers can still be used in the materials of construction.
Providing temperature control for the refrigeration systems discussed in this patent requires special techniques and design considerations. This is because the refrigerant mixtures used for very low temperatures are comprised of components have a wide difference in their normal boiling points. A difference of 120C or more is not uncommon between the normal boiling point of the warmest and coldest boiling point components. For such refrigerant mixtures there is no fixed relationship between evaporating temperature and pressure. With such refrigerants, a range of evaporating temperatures are possible for a given evaporating pressure. In addition, the materials and details of construction do not allow these standard pressure regulating valves to be used at the outlet of a very low temperature evaporator.
U.S. Pat. No. 5,715,694, xe2x80x9cRefrigerator controller,xe2x80x9d assigned to Matsushita Electric Industrial Co., Ltd. (Osaka, Japan), describes a refrigerator controller that employs a non-azeotropic mixture refrigerant and a compressor, a four-way valve, an outdoor heat exchanger, a fractionator, an overhead condenser provided on the top of the fractionator, a first flow control valve, a throttle, and an indoor heat exchanger which are connected in the form of a ring. The controller further employs a circuit at a lower part of the fractionator returning to the bottom of the fractionator through a second flow control valve and a reheater, and a circuit returning from the overhead condenser to the top of the fractionator, thereby controlling the openings of the first and second flow control valves by the output signal of indoor and outdoor heat exchanger temperature detectors, an indoor temperature detector, and an indoor temperature setting device through a flow controller. In this manner, a wider control range of refrigerating capability can be realized. Thus a high separation effect for mixed refrigerant components is obtained while controlling a safe and optimum refrigerant cycle.
U.S. Pat. No. 6,233,955, xe2x80x9cIsothermal coolant circulating apparatus,xe2x80x9d assigned to SMC Corporation (Tokyo, Japan), describes an energy-saving isothermal coolant circulating apparatus capable of reducing the cost of operation and equipment. A refrigerating circuit section comprises a hot gas circuit for mixing high-temperature refrigerant discharged from a compressor with low-temperature refrigerant being supplied to an evaporator via an electronic expansion valve, and an electronic expansion valve in the hot gas circuit. Coolant returned from a load to a coolant circuit section is cooled in a heat exchanger incorporated by the evaporator thereunto and then mixed with coolant returned from the load in an electric-motor-operated three-way valve to a predetermined temperature. An expansion valve controller in a controlling section controls the electronic expansion valves, while a temperature controller controls the electric-motor-operated three-way valve therein.
U.S. Pat. No. 6,167,722, xe2x80x9cRefrigeration unit,xe2x80x9d assigned to Hitachi, Ltd., (Tokyo, Japan), describes a refrigeration unit using HFC group coolant, having an enhanced cooling capacity so that the performance and the performance coefficient thereof can be enhanced, and the operation thereof can be stably made. The refrigeration unit has a refrigerating cycle with a compressor, a condenser, an expansion valve, and an evaporator. The expansion valve is connected in series in the mentioned order. The unit also comprises HFC group coolant, and a subcooler located between the condenser and the evaporator, wherein the HFC group coolant is turned into liquid coolant in the subcooler, and is then branched into a main liquid stream and a substream which super-cools the main liquid stream through a super-cooling expansion valve, and is led into the intermediate stage part of the compressor.
The present invention provides the use of a standard evaporator pressure regulating valve in a very low temperature refrigeration system that uses nonstandard refrigerant mixtures. The refrigerant mixtures are comprised of at least two components with normal boiling points that differ by at least 50 C. The use of this standard valve with the nonstandard refrigerant mixture provides control of the coldest temperature of the refrigerant system to keep the temperature from becoming colder than desired.
One advantage of the current invention is that standard refrigeration control valves are used to provide temperature control of a refrigeration system that uses a refrigerant mixture comprised of at least two refrigerants with normal boiling points that differ by at least 50 C.
A second advantage of this invention is that the standard refrigeration control valve does not experience the very low temperatures produced by the refrigeration system.
A third advantage of the invention is that the object being cooled by the refrigeration system is prevented from becoming colder than desired.
A fourth advantage of the invention is that the temperature of the refrigerant mixture is controlled so that none of the individual refrigerants freeze out from the refrigerant mixture.
Still other objects and advantages of the invention will be apparent in the specification.
The invention accordingly comprises the features of construction, combinations of elements, and arrangements of parts, which will be exemplified in the constructions hereinafter set forth, and the scope of the invention will be indicated in the claims.